Sheet Metal Forming. Part 1

Transcription

1 Sheet Metal Forming Part 1

2 Sheet Metal Forming For products with versatile shapes and lightweight Dates to 5000 B.C. Products include metal desks, file cabinets, appliances, car bodies, beverage cans Common materials: low carbon steel, aluminum or titanium First take sheet plate and cut into pieces by shearing, slitting, cutting, or sawing or produce from coil Then form into shapes by punching, blanking, stamping, embossing, bending, forming, deep drawing, and a variety of other processes

3 TABLE 16.2 Characteristic Elongation Yield point elongation Anisotropy (planar) Anisotropy (normal) Grain size Residual stresses Springback Wrinkling Quality of sheared edges Surface condition of sheet Sheet Metal Characteristics Importance Determines the capability of the sheet metal to stretch without necking and failure; high strain hardening exponent (n)and strain rate sensitivity exponent (m)desirable. Observed with mild steel sheets; also called Lueder s bands and stretcher strains; causes flamelike depressions on the sheet surfaces; can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling. Exhibits different behavior in different planar directions; present in cold rolled sheets because of preferred orientation or mechanical fibering; causes earing in drawing; can be reduced or eliminated by annealing but at lowered strength. Determines thinning behavior of sheet metals during stretching; important in deepdrawing operations. Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher the appearance (orange peel); also affects material strength. Caused by nonuniform deformation during forming; causes part distortion when sectioned and can lead to stress corrosion cracking; reduced or eliminated by stress relieving. Caused by elastic recovery of the plastically deformed sheet after unloading; causes distortion of part and loss of dimensional accuracy; can be controlled by techniques such as overbending and bottoming of the punch. Caused by compressive stresses in the plane of the sheet; can be objectionable or can be useful in imparting stiffness to parts; can be controlled by proper tool and die design. Depends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work hardened layer, which are all detrimental to the formability of the sheet; quality can be improved by control of clearance, tool and die design, fine blanking, shaving, and lubrication. Depends on rolling practice; important in sheet forming as it can cause tearing and poor surface quality; see also Section 13.3.

4 Formability Formability is the ability of sheet metal to undergo shape change without failure by necking (a) or tearing Cupping (Swift or Ericson) tests give some idea of formability (a) Yield point elongation in a sheet metal specimen. (b) Lueder's bands in a low carbon steel sheet. Source: Courtesy of Caterpillar Inc. (c) Stretcher strains at the bottom of a steel can for household products. (a) (b) (c)

5 Forming Limit Diagrams (FLD) Sheet metal is marked with small circles, stretched over a punch, and deformation is observed in failure areas FLD shows boundary between safe and failure zones (a) Strains in deformed circular grid patterns. (b) Forming limit diagrams (FLD) for various sheet metals. Although the major strain is always positive (stretching), the minor strain may be either positive or negative. In the lower left of the diagram, R is the normal anisotropy of the sheet, as described in Section Source: S. S. Hecker and A. K. Ghosh.

6 Shearing A blank is a properly sized piece of sheet metal removed from a much larger sheet or coil by shearing Shearing is cutting by subjecting a workpiece to shear stresses Shearing starts with small cracks at points A, B, C, D which eventually grow and meet Rough fracture surfaces and smooth burnished surfaces result Shear angles or beveled edges often used on shearing dies (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug. Note that the scales of the two figures are different.

7 Shearing Parameters Clearance, c, between the punch and die typically between 2% and 10% of sheet metal thickness As clearance increases, sheared edge becomes rougher and zone of deformation becomes larger clearances are smaller for softer metals, thinner sheets, or larger holes Ratio of burnished to rough edges increases with: increasing ductility, decreasing clearance and thickness Faster punch speeds cause narrower sheared zones and less burr formation Burr height increases with increasing clearance, ductility, or dull tools Maximum Punch Force, F = 0.7 T L (UTS) (product of thickness, sheared edge perimeter, and UTS)

8 Shearing Operations Punching sheared slug is discarded Blanking slug is workpiece, surrounding area discarded Die cutting includes perforating (many holes), parting (separating into multiple pieces), notching (removing pieces from the edges), and lancing (leaving a tab) Fine blanking with 1% clearances produces very smooth and squared off edges (a) (b) (a) Comparison of sheared edges produced by conventional (left) and by fine blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. Source: Feintool U.S. Operations.

9 Shearing Operations Slitting cutting off with 2 circular blades (can opener) Steel rules a die for shearing soft metals, paper, leather, and rubber into specific shapes (cookie cutter) Nibbling reciprocating die for successive, overlapping holes that shears intricate, flexible shapes Shaving trims excess material to clean sheared edges Compound and progressive dies perform several operations Slitting with rotary knives. Shaving and compound dies

10 Other Methods of Cutting Sheet Metal Band saw metal material removal process that produces chips as in other machining Flame cutting especially for thick steel plates, as in shipbuilding Laser beam cutting newer process used with computer controlled equipment Plasma cutting high energy plasma formed by electric arc between tool and work material Friction sawing disk or blade that rubs against sheet or plate at high speeds Water jet cutting for metallic and non metallic workpieces

11 Laser Cutting

12 Mechanical Stamp Press

13 Sequential Process Steps

14 Progressive Die Work

15 Cut Off Die

16 Cut Off Operation Design parts with straight parallel edges and jig saw ends minimizes scrap and can be produced on simplest cut off die

17 Part Off Die

18 Part Off Operation Design parts with straight parallel edges reduces edge scrap and requires simpler part off die

19 Blanking Die

20 Hole Punching Die

21 Blank and Punch Die

22 Multi stage Stamping

23 Progressive Die

24 Processing Limits minimum hole diameters

25 Critical Dimensions in Design of Sheet Metal Blank * * * * * * * * All dimensions > 2 x gage thickness

26 Feature Position Limits * * Dimension > 4 x gage thickness

27 Reducing Scrap Scrap metal can be as high as 30% Computer aided design and planning can minimize scrap Tailor welded blanks are multiple pieces of flat sheet buttwelded together and simultaneously stamped Production of an outer side panel of a car body, by laser butt welding and stamping. Source: After M. Geiger and T. Nakagawa.

28 Reducing Waste Material